1. Field of the Invention
This invention relates to a digitally implemented control system for a motor, and particularly to a low cost, flexible, multi-loop, digital system fo controlling one or more dc motors powered from an ac power source.
2. Background Information
A common type of control system for a dc motor utilizes a thyristor power module to gate selected portions of half cycles of power from an ac source to the motor armature. Typically, a second thyristor power module controls the field current.
Gating signals for the thyristor power modules have traditionally been generated by analog control circuits. These analog control circuits for the thyristor power module energizing the armature, typically have had an inner loop regulating armature current and an outer loop regulating a second parameter such as speed or torque. Some systems have multiple inner loops for controlling energization of the armature. These additional loops provide alternative voltage control and current limits, for instance, in addition to current control. As the loops are feedback loops, each of the control signals represents an error in the control parameter. The largest control or error signal is selected for use in generating the firing signals for the thyristor power module. Typically, control action, such as proportional plus integral control, is applied in the control loops. Traditionally, each loop has its own integrator. While only one inner loop is effective at a time, the individual integrators of the non-selected loops are each made to track the output of the active integrator, so that bumpless transfer can be made between loops.
Under low load conditions, the converter phases back gating of voltage pulses to the armature. With this phase back, a point is reached where the current applied to the motor becomes discontinuous. As the gain of the converter changes when the current is discontinuous, gain compensation must be applied to the current controller under these conditions. A common method of detecting discontinuous current relies upon monitoring the average current in the armature circuit. The system is tuned up, and when the average current drops below a selected minimum value, it is presumed that the discontinuous current condition exists and the current controller gain compensation is applied. However, the average current is not an accurate determinant of discontinuous current.
The development of powerful, low cost, microprocessors has lead to the digital implementation of the control schemes traditionally carried out by the analog control circuits of dc motor control systems. However, as far as known, these digital systems still output an analog firing signal to the thyristor power module.